image format matters

The guideline suggests PNG with an sRGB color profile is the recommended format for app icons. It may convert the icons without the sRGB profile, but you might notice errors in the colors if you're checking close enough.

Secondly, and this is the one that tripped me up: The png file likely requires the alpha flag.

Either that, or the first one it finds is what it tries to use for every other one, and if they differ it will fail without a clear error.

This sounds obscure, so here is how I found what it wanted.

Open the file in preview

Press ⌘I (cmd+I) or Tools -> Show Inspector

Click the i icon and under the general tab, compare the images.

I noticed the first bunch said "Has Alpha" and the last one didn't.

Once I added the alpha channel it worked again (even though all my icons were already matching and working previously).